The present invention relates to a heat- and flame-resistant conducting sheet which is suitable for electrostatic discharge interference and/or electromagnetic interference shielding or for covering a source electronic device, particularly for use in proximity to said device, and which has a highly heat- and flame-resistant self-extinguishing electrically insulating layer; and a manufacturing process thereof.
Recent developments in the microelectronic technology have resulted in a broad range of electronic devices that are sensitive to external electromagnetic waves and that themselves transmit unwanted electromagnetic waves. In addition, an extensive use of plastics in electronic devices makes them easily statically charged in contact with an electrical insulator to readily cause electrostatic and electromagnetic interferences. In addition, they themselves are vulnerable to electromagnetic interference, causing these devices to behave erratically.
Measures to attenuate these electromagnetic and electrostatic charge-discharge interference problems include using a variety of conducting sheets. Particularly, when a conducting sheet is used for electromagnetic interference shielding, it is often located in close proximity to or in contact with an electrical circuit acting as a source of such electromagnetic waves. This manifests a danger of the conducting sheet coming into contact with the electrical circuit, thereby electrically short-circuiting. That is why it is essential for the surface of an electromagnetic shield material and an electromagnetic shielding housing to be electrically insulating. There is also a fire hazard if an electrical shortening occurs. In addition, the generation of heat from the electrical circuit requires using a highly heat- and flame-resistant conducting sheet for preventing fires. For example, Japanese Patent Application Publication Kokai 51-47103 and 57-115702 teach a heat resistant conducting paper based on aromatic polymer pulp and conducting fibers. A flame resistant or noncombustible structure is known in the art: for example, Kokai 63-209199 teaches a sheet-like electromagnetic shield structure in which metal foil is bonded through an adhesive to one side of a sheet-like structure, for example, paper or nonwoven fabrics. Kokai 49-94904 teaches a sheet obtained by treating with heat and pressure a mixture of aromatic polyamide pulp granules with short aromatic polyester fibers, inorganic fibers, and the like. A heat- and flame-resistant paper is, for example, taught in Kokai 1-132898: a heat- and flame-resistant paper composed of a heat resistant polyether imide resin.
However, the above prior art sheet materials used for electrostatic or electromagnetic interference shielding are deficient, particularly when used in close proximity to an electronic device acting as a source of electrostatic or electromagnetic interference. For example, a conducting paper based on aromatic polymer pulp and conducting fibers has the drawback of short circuiting the electronic device because the conducting fibers tend to drop off. In addition, a composite sheet material obtained by bonding metal foil by an adhesive to one side of paper or nonwoven fabric is deficient in that it lacks bending characteristics, is poor in heat resistance, and shows reduced heat- and flame-resistant due to the use of adhesives. Furthermore, a composite product obtained by bonding with an adhesive a conducting sheet to a heat- and flame-resistant aromatic polyamide or polyether imide paper suffers from a loss in heat resistance and flame resistance due to the presence of an adhesive, which makes such use a hindrance in the formation of a heat- and flame-resistant conducting sheet.
The present inventors have studied to develop a highly heat- and flame-resistant and electrically insulating conducting sheet for solving the above prior art problems, and as a result, they have arrived at this invention.